Thermal conversion of blended food production waste and municipal sewage sludge to recoverable products

Abstract

Renewable fuels are necessary energy sources as bio-based combustible fuels can be utilized in existing infrastructure while lowering the dependence on petrochemical based fuels. Current bio-fuels are predominantly crop-based which require tillable land, fertilizer, chemicals, irrigation, and fuel to cultivate the biomass feedstock. This study investigated the hydrothermal conversion of underutilized waste streams for biomass feedstock. This work blended municipal sewage sludge solids, poultry dissolved air flotation solids, and used cooking oil to form a waste slurry that was thermally converted to renewable fuel products. Blended waste processing provides flexibility and robustness in the future of bio-based fuels, and the authors believe this is the first documented hydrothermal processing of this specific blended waste. The waste slurry was heated under subcritical hydrothermal conditions, where solids were separated, and then heated in a second pressurized reaction to complete the thermal conversion process of wastes to recoverable products. Prior to processing, the blended waste slurry contained 8,490 kJ/kg (3,650 BTU/lb). Following processing, the recovered gas products contained 38,747 kJ/kg (16,658 BTU/lb) and 42% methane. The recovered renewable oil contained 40,468 kJ/kg (17,398 BTU/lb) and 0.012% ash. The effluent produced water was appraised for nutrient content and was found to contain 1,689 mg/L of nitrogen, which has potential as a liquid fertilizer product. The produced gas fuel, produced liquid fuel, solid char, and produced nutrient laden water were determined to be potential marketable products.